US 6516291 B2 Abstract A circuit that provides the root-mean-square (RMS) value of an input signal and that detects and independently recovers from an output fault condition is provided. The circuit includes reconfigurable circuitry that changes from normal operating mode to fault recovery mode when an output fault is detected. During fault recovery mode, the circuit provides a modified output signal that allows independent recovery from an output fault condition. Once recovery is complete, the circuit returns to normal operating mode and provides a DC output signal proportional to the RMS value of an AC input signal.
Claims(15) 1. In a circuit that generates an output signal at an output node proportional to a root-mean-square (RMS) value of an input signal, a method for detecting and recovering from an output fault condition, the method comprising:
monitoring the circuit for an occurrence of an output fault; and
generating a modified output signal based on the input signal when the output fault condition is detected so that said modified output signal, when present at the output node, causes the circuit to recover from the fault condition.
2. The method of
3. The method of
4. The method of
5. The method of
reconfiguring at least a portion of the circuit to create said modified output signal; and
providing at least a portion of a signal present at the output node back to the circuit as a feedback signal.
6. The method of
7. The method of
providing the input signal to said absolute value detect circuit; and
performing an absolute value operation on the input signal.
8. The method of
9. The method of
10. In a circuit that generates a direct current (DC) output signal at an output node proportional to a root-mean-square (RMS) value of an input signal, a method for detecting and recovering from an output fault condition, the method comprising:
providing an analog input signal to the circuit;
providing at least a portion of the DC output signal back to the circuit as a feedback signal;
generating a first pulse code modulated representation of said analog input signal and said DC output signal with a reconfigurable circuit;
monitoring said first pulse code modulated signal for an occurrence of an output fault with a monitor circuit; and
generating a modified output signal based on the input signal when the output fault condition is detected so that said modified output signal, when present at the output node, causes the circuit to recover from the fault condition.
11. The method of
12. The method of
13. The method of
14. The method of
15. The method of
generating a second pulse code modulated representation of said analog input signal and said output signal with a pulse with modulator circuit;
reconfiguring said reconfigurable circuit to function as a comparator circuit when the output fault condition is detected, said comparator circuit producing an output indicative of the polarity of the input signal;
monitoring said second pulse code modulated representation of said input signal and said output signal to determine when the circuit recovers from the output fault condition; and
reconfiguring said reconfigurable circuit to function as a pulse code modulator circuit when said monitor circuit determines that the circuit has recovered from the output fault condition so that the circuit produces the (DC) output signal proportional to the root-mean-square (RMS) value of the input signal.
Description The present invention relates to apparatus and methods for providing an output signal proportional to the root-mean-square (RMS) value of an input signal. More particularly, the present invention relates to apparatus and methods for detecting an output fault condition and for recovering from such a condition so that an output signal is provided. The output signal may be a direct current (DC) signal proportional to the RMS value of an input signal (commonly called RMS-to-DC conversion). The RMS value of a waveform is a measure of the heating potential of that waveform. RMS measurements allow the magnitudes of all types of voltage (or current) waveforms to be compared to one another. Thus, for example, applying an alternating current (AC) waveform having a value of 1 volt RMS across a resistor produces the same amount of heat as applying 1 volt DC voltage across the resistor. Mathematically, the RMS value of a signal V is defined as:
which involves squaring the signal V, computing the average value (represented by the overbar in equation (1)), and then determining the square root of the result. Various previously known conversion techniques have been used to measure RMS values. One previously known conversion system uses oversampling analog-to-digital converters to generate precise digital representations of an applied signal. The digital representations are demodulated and filtered to produce a DC output signal that has the same heat potential as the applied signal. This type of system is attractive to circuit designers because it produces highly accurate results and can be efficiently implemented on an integrated circuit. FIG. 1 is a generalized schematic representation of a portion of an RMS-to-DC converter circuit. As shown in FIG. 1, RMS-to-DC converter circuit To simplify the description of pulse modulator Integrator Non-inverting buffer V The first and second inputs of integrator
and V From equation (2), if V That is, if V Demodulator If V Demodulator Substituting equation (3) into equation (4b), V Lowpass filter Gain stage Thus, circuit Demodulator Because the output of comparator With a DC input, this may not be problematic, because the state of V Thus, in view of the foregoing, it would be desirable to provide methods and apparatus for performing RMS-to-DC conversions that have improved recovery characteristics. Accordingly, it is an object of this invention to provide methods and apparatus for performing RMS-to-DC conversions that have fault detection and recovery capabilities. In accordance with this and other objects of the present invention, circuitry and methods that supply the root-mean-square (RMS) value of an input signal and that detect and independently recover from output fault conditions are provided. The circuit of the present invention includes reconfigurable circuitry that changes from normal operating mode to fault recovery mode when an output fault is detected. During fault recovery mode, the circuit of the present invention generates a modified output signal that allows independent recovery from an output fault condition. Once recovery is complete, the circuit returns to the RMS mode of operation. The above-mentioned objects and features of the present invention can be more clearly understood from the following detailed description considered in conjunction with the following drawings, in which the same reference numerals denote the same structural elements throughout, and in which: FIG. 1 is a schematic diagram of a previously known RMS-to-DC converter circuit; FIG. 2A is a schematic diagram of an RMS-to-DC converter circuit of the present invention; FIG. 2B is another schematic diagram of an RMS-to-DC converter circuit of the present invention; FIG. 3A is another schematic diagram of an RMS-to-DC converter circuit of the present invention; FIG. 3B is another schematic diagram of an RMS-to-DC converter circuit of the present invention; FIG. 4A is another schematic diagram of an RMS-to-DC converter circuit of the present invention; FIG. 4B is another schematic diagram of an RMS-to-DC converter circuit of the present invention; FIG. 5 is a schematic diagram of the reconfigurable ΔΣ modulator of FIGS. FIG. 2A illustrates an embodiment of RMS-to-DC converter constructed in accordance with the principles of the present invention. Circuit Pulse modulator Reconfigurable ΔΣ stage ΔΣ stages where index i denotes the sample index and e[i] is the quantization error of reconfigurable ΔΣ stage ΔΣ stage where e′[i] is the quantization error of ΔΣ stage where
Delay stage V Non-inverting buffer V Subtractor Thus, V
Output V
The circuit of FIG. 2A may be implemented using single-ended or differential circuitry. During operation, output signals from reconfigurable ΔΣ stage The number of consecutive same logic level bits that constitute a fault condition may be varied if desired. For example, with certain modulator topologies, the number of bits may be set to be relatively long (e.g., about 50) to ensure circuit In fault recovery mode, switch With this arrangement, shown in FIG. 2B, circuit As shown in FIG. 2B, to operate as a mean-absolute-detect circuit, the feedback from V When the output of comparator As long as output signal V When circuit The overall gain of circuit FIG. 3A shows another illustrative embodiment of RMS-to-DC converter constructed in accordance with the present invention. Converter Reconfigurable ΔΣ stage In addition, V ΔΣ stage Single-bit DACs
Adder/subtractor
which equals (assuming gain B=1): Note that: If the time constant of lowpass filter which may be written as: The first term on the right side of equation (26) is the desired output, and the second term equals the second-order spectrally-shaped quantization noise of ΔΣ stage Output V
The circuit of FIG. 3A may be implemented using single-ended or differential circuitry. During operation, output signals from reconfigurable ΔΣ stage In fault recovery mode, switch In this arrangement, shown in FIG. 3B, circuit As shown in FIG. 3B, to operate as a mean-absolute-detect circuit, the feedback from V When the output of comparator As long as output signal V When circuit The overall gain of circuit FIG. 4A illustrates another embodiment of RMS-to-DC converters constructed in accordance with the principles of the present invention. Circuit Modulator Reconfigurable ΔΣ stage ΔΣ stage where e′[i] is the quantization error of ΔΣ stage where
V V The digital differentiator formed by delay stage Subtractor Thus, V
and output V
The circuit of FIG. 4A may be implemented using single-ended or differential circuitry. During operation, output signals from reconfigurable ΔΣ stage In fault recovery mode, switch In this arrangement, shown in FIG. 4B, circuit As shown in FIG. 4B, to operate as a mean-absolute-detect circuit, the feedback from V When the output of comparator As long as output signal V When circuit The overall gain of circuit As mentioned above, monitoring circuit One such case is when the amplitude of the input signal (V Another case during which a fault condition may be detected is when input signal V FIG. 5 is a schematic diagram of one possible embodiment of reconfigurable ΔΣ stage When configured as ΔΣ stage Input voltage V Closing switches In the integration phase, switches When configured as comparator stage Input voltage V In the sample and hold phase, switches Persons skilled in the art will recognize that the apparatus of the present invention may be implemented using circuit configurations other than those shown and discussed above. All such modifications are within the scope of the present invention, which is limited only by the claims that follow. Patent Citations
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